1. Field of the Invention
This invention relates generally to liquid level gauging systems, and more particularly to devices of this type wherein coupled light-emitting diodes and transducers are immersed in vessels associated with internal combustion engines.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
The present invention involves improvements in the gauging system disclosed and claimed in the U.S. application above identified.
The gauge of this application employed a probe carrying a light-emitting diode which was excited from direct current, and a photoransistor disposed adjacent the light-emitting diode and arranged to receive refracted light therefrom through a lens of the probe. The probe was immersible in oil contained in the oil pan or crankcase of the vehicle.
While the disclosed circuit was considered to operate in a generally satisfactory manner, it was desired to improve its sensitivity. Initially, it was believed that such an increase could be achieved by substituting for the single stage phototransistor, a Darlington phototransistor. The added current gain did indeed provide the desired increased sensitivity, and was acceptable for relatively low temperature operation.
Where the gauge was employed to monitor the level of oil in crankcases, however, wide excursions of temperature were encountered, and at the high end of the expected temperature range, well above 100.degree. C., the Darlington phototransistor exhibited leakage currents which tended to interfere with its proper operation, and to mask the measurements being made. As presently understood, the problem was a result of leakage current originating at the base-collector junction of the first stage of the Darlington phototransistor, feeding directly into the base of the second stage, where it was amplified thereby.
This problem associated with leakage was obviated by reverting to the use of a single stage phototransistor, as originally employed. However, there still existed the need to increase the sensitivity of the system and reduce the effect of heat, preferably without resorting to additional amplifiers or complex circuitry.